Sen. Roger Wicker has been sent an envelope containing a substance that tested positive for ricin.

Several senators around 6:30 p.m. Tuesday confirmed to CQ Roll Call the existence of the contaminated letter. Shortly thereafter, several senators reported that the letter was intercepted in the screening process at the Senate Mail Handling Facility and it never made it to the Capitol itself.

After 7:30 p.m., Senate Sergeant-at-Arms Terrance W. Gainer verified the information in an internal memo to his employees.

“The exterior markings on the envelope in this case were not outwardly suspicious, but it was postmarked from Memphis, Tennessee and had no return address,” Gainer wrote, adding that the Senate’s off-site mail processing center would be closed “for the next two to three days while testing and the law enforcement investigation continues.”

The Capitol Police, the FBI and “other agencies” are coordinating on the investigation, Gainer said.

In a separate memo, House Sergeant-at-Arms Paul Irving alerted his chamber about the incident, while adding that the off-site House Mail Handling Facility had not as of yet received any suspicious mailings.

In an email to CQ Roll Call, Gainer emphasized that “the letter in question never entered any Senate office facility or building other than our mail processing facility.

“Our mail screening procedures worked; the letter was discovered, processed tested and never reached its intended target,” he continued.

Around 8:35 p.m., Wicker’s office sent out a statement, saying, “This matter is part of an ongoing investigation by the United States Capitol Police and FBI. I want to thank our law enforcement officials for their hard work and diligence in keeping those of us who work in the Capitol complex safe.”

Though Capitol Hill law enforcement officials did not immediately release public statements or return media inquiries, news happened to break of the ricin-contaminated envelope while senators were already in a closed briefing with Homeland Security Secretary Janet Napolitano and FBI Director Robert Mueller regarding the investigation of Monday’s bombing at the Boston Marathon.

Senators were then informed of the letter, although they were not told the identity of their colleague who was targeted, many said. As they exited the briefing, many lawmakers were willing to speak generally with reporters about what they had learned so far.

Though they said the news was troubling, lawmakers did not appear to be particularly worried.

“Everything that is sent to us is sliced, everything that is mailed to us is roasted, toasted, sliced and opened,” Senate Majority Whip Richard J. Durbin, D-Ill., said, “so by the time it reaches us it doesn’t look like the original letter.”

In the aftermath of the Sept. 11, 2001, terrorist attacks, the Senate received letters containing Anthrax; three years later, the chamber was targeted with letters laced with ricin. Those incidents changed mail processing protocols considerably.

Capitol Hill and the rest of the Washington region are also on high alert following the Monday attacks in Boston, with law enforcement going so far as to take Capitol complex trash cans out of commission for fear they could be used as vessels for explosives.

Durbin declined to draw conclusions about whether the incidents are related.

“I don’t know if it’s a coincidence, it’s too early to tell. We don’t know enough about Boston to even speculate.”

“I think they’ve taken real good precautions to notify everybody as to what happened, and what has happened was far enough away and was caught in a way that it did no damage,” Michigan Democratic Sen. Carl Levin added. “I’m not excessively concerned about it. I think they’ve taken strong steps to avoid any problem.”

“Postal officials and law enforcement did an excellent job in detecting and preventing this threat before it reached the Capitol,” said in an email to CQ Roll Call. “The protective measures worked.”

The Centers for Disease Control and Prevention describes ricin as “a poison ... [that] can be made from the waste material left over from processing castor beans [and] can be in the form of a powder.”

“Ricin poisoning is not contagious,” the CDC continues, but one can become exposed to “ricin-associated illness” if he or she comes in contact with someone who has the material on their body or their clothes, and could be fatal if not responded to quickly.

Though comforting to lawmakers and staff that the envelope of dangerous material never made it inside the Capitol complex, it was not immediately known whether anyone at the mail processing center was exposed and receiving treatment.

Gainer told CQ Roll Call that while the mail handlers were wearing proper attire, “they were all decontaminated and provided clean clothing where needed.” None were exhibiting any symptoms, he said, and they were all now at home.

“These men and women are very quiet professional sentinels,” he wrote in an email. “They intercepted a dangerous letter [meant] to hurt others.”

For senators’ state offices without the first line of defense of a government-run mail processing facility to safeguard against contaminated mail, Gainer said all headquarters outside Washington, D.C. should use “Postal Sentry,” a mail processing system available to state offices free of charge that can help “contain potentially harmful agents.”

Calling it “the only defense available in state offices for this type of threat, Gainer went on to describe the Postal Sentry as “a lightweight desktop device that provides sufficient airflow and filtration to reduce the release of potentially harmful particles while opening mail in an office setting.”

This is the first time in almost a decade that Capitol Hill has been targeted with a genuinely harmful material. There have been periodic scares from time to time, however.

In January, Virginia Republican Reps. Scott Rigell and J. Randy Forbes received letters containing unidentified substances in their district offices.

And a year ago, a handful of House members and nearly all senators received mailings at their home state and Capitol Hill offices containing suspicious powders. In March 2012, the coordinated efforts of the Capitol Police, the FBI and the U.S. Postal Inspection Service culminated in the arrest of Christopher Lee Carlson at his home in Vancouver, Wash.